ICPT systems generally consist of two electrically isolated parts. The first is a trackway or primary conductive pathway which is usually supplied with a very low frequency (VLF; typically 5-50 kHz) current. The primary conductive pathway is usually elongate but can take a variety of different forms.
The second part, which is usually referred to as a pick-up, supplies an electrical load with power derived from the primary conductive path. The pick-up has a pick-up coil (i.e. at least a partial turn of conductive material) in which a voltage is induced by mutual magnetic induction between the pick-up coil and the primary conductive path. The pick-up coil which is tuned with a capacitor to augment the power transfer capability. The tuning may be achieved with a series capacitor or a parallel capacitor. The output from the tuned circuit is then rectified and fed to a controller such as that described in the specification of U.S. Pat. No. 5,293,308, which partially decouples the pick-up coil to control the power transfer and match the power taken from the pathway to that required by the load. In this way a single output voltage is produced and maintained.
However, in many circumstances more than one output voltage is required—for example the ICPT system may be required to produce a 300 V (or higher) output voltage for a motor drive and at the same time provide 24 V DC for independently driving other control circuitry. With a series-tuned pick-up system an extra output voltage is relatively easy to produce. A conventional series pick-up and controller system is shown in FIG. 1. Current I induces a mutually coupled voltage in pick-up inductance 100 which is tuned with capacitor 101. The resonating current produced is rectified by bridge rectifier 102 to charge a large capacitor 103. Switch S 104 in combination with diode 105 and inductor 106 act as a Buck converter to produce a controlled output voltage on the load capacitor 107 and the load resistor 108. In this circuit the input voltage to the bridge rectifier 102 is essentially constant and independent of the load current. Thus, a simple transformer T 209 may be connected as shown across the rectifier bridge 202 as shown in FIG. 2. This transformer allows this voltage to be isolated and scaled as required (by changing the transformer turns-ratio). A simple rectifier filter and regulator circuit Reg 210 using any of a number of well known techniques then allows an additional output voltage (eg 24 V DC) to be produced. Series tuned systems, however, have higher internal working voltages than parallel tuned systems. For an output voltage of 300 V DC with a tuned circuit Q of 10 the voltage across the tuning capacitor would be 3 kV. Thus, for high power applications series tuned circuits are not preferred.
For the simple parallel tuned circuit of FIG. 3 the voltages across the tuned components L 300 and C 301 are similar to the output voltage across resistor 307 so that very high voltages do not occur. The detailed operation of this circuit is well known from U.S. Pat. No. 5,293,308, the contents of which are incorporated herein by reference. The circuit allows the power transfer from the primary conductive path of the ICPT system to be matched to the power required by the load. The output power from the circuit is controlled by the duty cycle of the switch 304 operating at some switching frequency. If the switch 304 is closed for most of the switching cycle corresponding to a high duty cycle the average output power is reduced, and if the switch 304 is open for most of the duty cycle the average output power will increase. Thus, the output power may be controlled by adjusting the switch duty cycle. If the switch 304 is held permanently closed the pick-up circuit becomes essentially completely decoupled from the trackway and essentially no power is transferred to the pick-up coil from the trackway. In operation therefore the duty cycle of the switch is controlled so that the pick-up coil is partially decoupled and the power transferred from the trackway to the pick-up coil is matched to the power required by the load. As described in U.S. Pat. No. 5,293,308, this may be performed by detecting the voltage across the load, comparing the detected voltage with a nominal or required output voltage, and coupling or decoupling as necessary to alter the output voltage toward the nominal output voltage. However, the input voltage to the rectifier bridge B 302 falls to nearly zero on very light loads corresponding to very low power transfer, so that a voltage transformer connected across the rectifier bridge as shown in FIG. 2 cannot be used to produce an additional independent output voltage.
Another possible option for generating an independent output from the pick-up topology shown in FIG. 3 is to feed the first output to a switching converter or regulator to provide an additional output at the required output voltage. However, as mentioned above, the first output (i.e. that which supplies load 307 in FIG. 3) is typically required to be in the vicinity of 300V to 550V which means that a switching device capable of handling these voltage levels is required. Such devices are expensive, and are not generally available for voltages at the top end of this range. Another disadvantage with using the first output is that the high voltage needs to be maintained even when the load it supplies is in an “off” state.